Odd types in TypeScript

• TypeScript types

Can I define, for example, “with an id consisting of alphanumeric characters only” in TypeScript type information? src I saw that, and I wondered how to do that, so I looked it up and made a note of it.

I think this kind of need is, in essence, using existing String as a data structure in Structural typing TypeScript, but restricting the range of values, and wanting to give a different name to the set of values, or Nominal Typing.

I looked it up in Nominal Typing and found it right away. Nominal Typing - TypeScript Deep Dive

It is simple enough to introduce a member for type distinction and to eliminate structural incompatibility. I used this explanation to create an odd-numbered type.

ts

interface Odd extends Number {
  _OddBrand: boolean;
}
interface Even extends Number {
  _EvenBrand: boolean;
}
 
var odd: Odd;
var even: Even;
 
// Expected Type Errors
// odd = even;  // NG
// even = odd;  // NG
// odd = 1;  // NG
 
const assertOdd = (x: number): Odd => {
  if (x % 2 === 1) {
    // @ts-ignore
    x._oddBrand = true;
    // @ts-ignore
    const ret: Odd = x;
    return ret;
  }
  throw AssertionError;
};
 
var x = assertOdd(1);
odd = x;  // OK
// even = x;  // NG

Some people may find it “tougher than expected”. odd = 1 results in a type error. As in this example, if you think “it’s a number, but it’s obviously odd,” you can just assert it. If this ASSERT is incorrect, an assertion error will occur at runtime.

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